Certain chemical compounds have long been known in the art to possess the desired quality of imparting softness to textile fabrics The quality of "softness" or being "soft" is well defined in the art, and, as used herein, means that quality of the treated fabric whereby its handle or texture is smooth, pliable, and fluffy, and not rough or scratchy to the touch. Known generally as "fabric softeners," these compounds have long been used by homemakers in the laundry, and by the textile industry to soften a finished fabric.
Additionally, many of these compounds act to reduce the "static cling" of the treated fabrics. Static cling is generally the phenomenon of a fabric adhering to another object or to parts of itself as a result of static electrical charges located on the surface of the fabric. It can also cause the adherence of lint, dust, and other undesired substances to the fabric. It is noticeably present in unsoftened fabrics that are freshly washed and dried in an automatic hot air dryer. By softening and reducing the static cling of a fabric, it is more comfortable when worn. Such treated fabrics additionally are easier to iron, and have fewer hard-to-iron wrinkles.
Perhaps the most common fabric conditioners known in the art are cationic compounds, especially amines such as quaternary ammonium and imidazolinium salts. These compounds are widely marketed for home use in the form of liquid emulsions. They must be added to the laundry in the rinse cycle, not the wash cycle, because cationic fabric conditioners interact with substances present in laundry detergents such as anionic surfactants and builder salts, thereby rendering both relatively ineffective. A commercial fabric conditioner of this type is Downy.RTM. (The Procter & Gamble Company, Cincinnati, Ohio).
Another means of providing fabric conditioning is disclosed in Gaiser, U.S. Pat. No. 3,442,692, issued May 6, 1969, incorporated herein by reference, comprising a fabric-conditioning composition in conjunction with a dispensing means for use in a hot air dryer. Preferred articles had the fabric-conditioning composition releasably affixed to an absorbent substrate, such as a nonwoven tissue, in the form of an impregnate or coating of cationic fabric-conditioning agent. The use of certain polyesters, especially sorbitan esters as auxiliary fabric-conditioning agents in products of this kind, is disclosed in Zaki et al., U.S. Pat. No. 4,022,938, issued May 10, 1977, incorporated herein by reference. A commercial product that has utilized the teachings of Gaiser and Zaki et al. is Bounce.RTM., The Procter & Gamble Company.
Substrates having fabric-conditioning agents adhered to substrates formed from natural or synthetic organic polymers have also been disclosed. For example, Schulz et al., U.S. Pat. No. 4,557,852, disclose a water-soluble sheet formed from a synthetic acrylate-type polymer which encloses a fabric softener or a bleach. This laundry care additive is added to the washing machine. Marshall et al., U.S. Pat. No. 3,936,538, disclose a fabric-softening composition for use in the dryer consisting of a sheet of a film-forming polymer having a molecular weight of at least 100,000, a fabric softener and a surfactant. However, these compositions leave a "crumpled sheet residue behind" in the dryer.
Compa et al. (U.S. Pat. No. 4,041,205) discloses a fabric softening sheet which is composed of 10-99% of the fabric softening agent which is reinforced with 1-40% of cellulose, microcrystalline protein or nylon fibers so that it maintains its integrity prior to addition to the laundry in the dryer. Compa et al. also disclose that the sheets can be reinforced with gums or solid organic polymers. These necessarily release large amounts of residue, since neither the fibers nor the polymeric reinforcing agents will evaporate during use.
Therefore, both the "absorbent substrate" and "all-chemical" type in-dryer softeners disclosed hereinabove can leave a powdered, solid or gummy residue or a residual base sheet which must be removed following the completion of the drying cycle. These sheets may be reusable to some extent, but the user has no way to readily determine whether or not sufficient softener is retained on the base sheet. Furthermore, although these products are easy to dispense, their efficacy depends on the efficient release of the fabric conditioner from a substrate which does not participate in the softening process, and which may itself decompose to soil the dried laundry. Also, in-dryer sheets generally do not soften as well as liquids, since the sheets may not contact all of the laundry evenly during the drying process. This can also lead to staining of the laundry due to the uneven release of the softener.
Commonly-assigned U.S. Pat. No. 4,938,879 and U.S. patent applications Ser. Nos. 7/521,242, filed May, 1990 and 7/568,836 filed Aug. 17, 1990 all disclose a sheet for in-dryer use formed by gelling a mixture of water and an organic solvent such as a glycol ether, with a fatty acid salt to form a dimensionally-stable matrix sheet. The sheet can act as a carrier for various fabric modifying agents such as quaternary amine fabric softening agents. When added to a rotary hot air dryer along with damp laundry, the sheet readily disintegrates as the matrix evaporates, leaving the laundry softened and no visible residue in the dryer.
Although these sheets are a marked improvement over earlier attempts to provide in-dryer softening, the use of large amounts of volatile organic solvent, e.g., 40-60% in the sheets disclosed in U.S. Pat. No. 4,938,879 can pose both an environmental risk due to the release of relatively large amounts of solvent. Also, such high-solvent sheets yield a gelled matrix that can be unacceptably brittle, thus increasing the processing and packaging difficulties associated with their manufacture, and making them difficult for the consumer to handle.
Therefore, there is a continuing need for a solid fabric softening sheet for use in an automatic hot air clothes dryer which is convenient to use, which softens effectively without leaving a residue in the dryer, and which does not stain or otherwise soil the dried laundry.